ct's Introduction

🧠 `ct.css` – Let’s take a look inside your `<head>`

Computed tomography of the head uses a series of X-rays in a CT scan of the head…
— wikipedia.org/Computed_tomography_of_the_head

Your <head> is the single biggest render-blocking part of your page—ensuring it is well-formed is critical. ct.css is a diagnostic CSS snippet that exposes potential performance issues in your page’s <head> tags.

Example Output

Red: This is an error and should be addressed.
Orange: This could be problematic in certain scenarios.
Green: This is fine and is purely informational.
Solid: This file is the problem.
Dashed: Another file(s) are causing problems with this file.

Simple Usage

Paste this anywhere in your HTML:

<link rel="stylesheet" href="https://csswizardry.com/ct/ct.css" class="ct" />

Chrome Snippet

Run Snippets Of JavaScript On Any Page With Chrome DevTools

(function(){
  var ct = document.createElement('link');
  ct.rel = 'stylesheet';
  ct.href = 'https://csswizardry.com/ct/ct.css';
  ct.classList.add('ct');
  document.head.appendChild(ct);
}());

Limitations

Non-matching media types: Currently, ct.css only takes print stylesheets into account. This means that you may experience false-positives for other non-blocking stylesheets.
Injected script elements: ct.css, as with all CSS, acts against the DOM and not the HTML. As such, injected script elements will be falsely flagged as blocking.
- Similarly, any script elements that are removed from the DOM will not be flagged.
Third-party blocking resources: The check for whether or not something is a third party is naive at best.
CSP Issues: If your app is blocking CSS from the csswizardry.com origin, either self-host the ct.css file, or paste its contents into <style class="ct">…</style> tags anywhere in your page.

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